NJIT Physics 320: Astronomy and Astrophysics – Lecture XIV Carsten Denker Physics Department Center for Solar–Terrestrial Research

December 10th, 2003NJIT Center for Solar-Terrestrial Research Pluto, Solar System Debris, and Formation  The Pluto-Charon System  Comets  Asteroids  Meteorites  The Formation of the Solar System

December 10th, 2003NJIT Center for Solar-Terrestrial Research The Pluto-Charon System  Pluto discovered by Clyde W. Tombaugh in 1930 (15 th magnitude)  yr orbital period  Eccentricity 0.25  29.7 AU perihelion (closer than Neptune)  49.3 AU aphelion  17° inclination to ecliptic  3-2 orbital resonance with Neptune (no danger of collsions)  Radius 1160 km  Its moon Charon discovered in 1978  Orbit around common center of mass in 6.4 d  Separation km (1/20 Earth-Moon distance)  Reduced mass is 0.24% mass of Earth  M Charon / M Pluto = 0.09 to 0.16  Orbital plane of Pluto- Charon system is inclined 122.5° with respect to their orbit around the Sun

December 10th, 2003NJIT Center for Solar-Terrestrial Research Pluto Pluto is mostly brown. No spacecraft has yet visited this most distant planet in our Solar System. The map was created by tracking brightness changes from Earth of Pluto during times when it was being partially eclipsed by its moon Charon. Pluto's brown color is thought dominated by frozen methane deposits metamorphosed by faint but energetic sunlight.

December 10th, 2003NJIT Center for Solar-Terrestrial Research Pluto-Charon Pluto is the only planet in our Solar System remaining unphotographed by a passing spacecraft. These maps depict the face of Pluto (left) that always faces Charon, and the face of Charon that always faces away from Pluto. The Pluto-Kuiper Express mission is tentatively planned for launch in 2004 and might encounter Pluto as early as 2012.

December 10th, 2003NJIT Center for Solar-Terrestrial Research Comets  Halley’s comet (observed since 240 B.C., 76 yr orbital period)  Nucleus: “dirty snowball” or “snowy dirtball”?  Size:  10 km  Coma: cloud of gas and dust, sublimated ice  Interaction with sunlight and solar wind creates dust (radiation pressure) and ion (magnetic field) tail up to 1 AU length  A hydrogen gas halo envelopes the coma  Tails are always directed away from the Sun (ion trails are straight, dust tails are curved)  Dust grains scatter light, tail appears white/yellow  Blue ion tail: CO + ions absorb UV radiation and reradiate at 420 nm  Composition: 80% H 2 O, 10% CO, 3.5% CO 2, few % (H 2 CO) n, 1% CH 3 OH

December 10th, 2003NJIT Center for Solar-Terrestrial Research Comets (cont.)  Disconnection events  Water on terrestrial planets from comet impacts?  Halley: Suisei, Sakigake, Vega 1/2, Giotto (closest approach 600 km), Cometary Explorer  Halley’s size 15 km  7.2 km  7.2 km  Mass: 5  kg to kg  Halley is a short-period comet < 200yr (Kuiper belt objects 30 AU to 100 AU)  Long-period comets 100,000 to 1 million yr  Long-period comets originate in the Oort cloud  Inner cloud in ecliptic 3,000 AU to 20,000 AU  Outer cloud has spherical distribution 20,000 AU to 100,000 AU  Planetesimals “catapulted” from Jovian planets to Oort cloud  Random motion

December 10th, 2003NJIT Center for Solar-Terrestrial Research Dust Tail R < R crit : net outward force, spiral away from Sun R > R crit : continue to orbit Sun (Poynting-Robertson effect!)

December 10th, 2003NJIT Center for Solar-Terrestrial Research Hale-Bopp

December 10th, 2003NJIT Center for Solar-Terrestrial Research Halley’s Comet

December 10th, 2003NJIT Center for Solar-Terrestrial Research Sungrazer (SoHO/LASCO)

December 10th, 2003NJIT Center for Solar-Terrestrial Research Asteroids  Minor planets mostly between Mars and Jupiter  Discovery of Ceres in 1801 by Piazzi  Combined mass of all asteroids 5  10  4 M   Orbital resonances with Jupiter  Kirkwood gaps  Trojan asteroids (1:1 resonance group, Lagrange points L4 and L5)  Hirayama families (originally single asteroid that suffered a catastrophic collision)  Collision speeds of up to 5 km/s  Composition is a function of the distance from the Sun (volatiles (water) vs. refractory compounds (silicon))  Metal rich asteroids from larger parent asteroids with chemical differentiation

December 10th, 2003NJIT Center for Solar-Terrestrial Research Asteroids (cont.)

December 10th, 2003NJIT Center for Solar-Terrestrial Research Orbital Resonances and Trojans

December 10th, 2003NJIT Center for Solar-Terrestrial Research Lagrange Points The Italian-French mathematician Lagrange discovered five special points in the vicinity of two orbiting masses where a third, smaller mass can orbit at a fixed distance from the larger masses. The Lagrange Points mark positions where the gravitational pull of two large masses precisely equals the centripetal force required to rotate with them.